In a fuel cell, electric current is generated with a high efficiency by the electrochemical combination of hydrogen (H2) and oxygen (O2) at an electrolyte to form water (H2O), and if the fuel gas used is pure hydrogen, there are not even any emissions of pollutants and carbon dioxide (CO2). Technical implementation of this principle of the fuel cell has lead to various solutions, specifically with different electrolytes and operating temperatures between 60° C. and 1000° C. The fuel cells are classified as low-temperature, medium-temperature and high-temperature fuel cells as a function of their operating temperature, and these classes can in turn be subdivided by virtue of having different technical embodiments.
An individual fuel cell provides an operating voltage of at most approximately 1.1 V. Therefore, a large number of fuel cells are stacked on top of one another to form a fuel cell stack which forms part of the fuel cell block. Connecting the fuel cells of the fuel cell block in series makes it possible to achieve operating voltages of a fuel cell block of 100 V and above.
A planar fuel cell includes a flat electrolyte, one flat side of which is adjoined by a flat anode and the other flat side of which is adjoined by a likewise flat cathode. These two electrodes together with the Electrolyte form what is known as an electrolyte-electrode assembly. An anode gas space is arranged adjacent to the anode, and a cathode gas space is arranged adjacent to the cathode.
Between the anode gas space of a fuel cell and the cathode gas space of a fuel cell which adjoins this fuel cell there is an interconnector plate. The interconnector plate produces an electrical connection between the anode of the first fuel cell and the cathode of the second fuel cell. Depending on the type of fuel cell, the interconnector plate is designed, for example, as an individual metallic plate or as a cooling element which includes two stacked plates with a cooling water space between them. Depending on the particular embodiment of the fuel cells, further components, such as for example electrically conductive layers, seals or pressure cushions, may be present in a fuel cell stack.
While they are operating, the fuel cells of a fuel cell block are supplied with operating gases, i.e. a hydrogen-containing fuel gas and an oxygen-containing oxidation gas. Some embodiments of low-temperature fuel cells, in particular fuel cells with a polymer electrolyte membrane (PEM fuel cells), require humidified operating gases for them to operate. These operating gases are saturated with steam in a suitable device, such as for example a liquid ring compressor or a membrane humidifier.
If the operating gases are passed through long operating gas feedlines from the humidifier to the fuel cell block, the temperature of a humidified operating gas may drop over this path as a result of heat being lost to the environment. This leads to condensation of humidification water. The operating gases are then heated again in the fuel cells, with the result that their relative humidity decreases. As a result, the electrolyte, which is always to be kept moist and is extremely sensitive to drying out, is damaged, with the result that its service life is shortened. It is therefore desirable for the humidifier to be arranged as close as possible to the fuel cells.
U.S. Pat. No. 5,200,278 and U.S. Pat. No. 5,382,478 have disclosed a fuel cell block having a stack of planar fuel cells and a stack of planar humidification cells. The two stacks are arranged directly adjacent to one another in the fuel cell block. The humidification cells are designed as membrane humidifiers with an operating gas space, a humidification water space and a water-permeable membrane arranged between the two spaces. Before the operating gases are fed to the fuel cells of the fuel cell stack, they flow through the humidification cells, where they are humidified, and then flow into the fuel cell stack without having to leave the fuel cell block.
In the humidification cells, the operating gases are humidified with the aid of the cooling water from the fuel cells. The cooling water, which has been heated to the temperature of the fuel cells, flows through the humidification water space, penetrates through the water-permeable membrane and then humidifies the operating gas to a degree of humidification of approximately 100%. However, the evaporation of the humidification water from the membrane into the gas space consumes heat from the operating gas as heat of evaporation. This reduces the temperature of the operating gas flowing through the gas space considerably. The operating gas, which has been humidified to approximately 100% in the humidification cells, is therefore heated again in the fuel cells, with the result that the degree of humidification of the operating gas drops and the electrolyte is attacked.